High voltage electrolytic capacitors are employed as energy storage reservoirs in many applications, including implantable medical devices. These capacitors are required to have a high energy density because it is desirable to minimize the overall size of the implanted device. This is particularly true of an implantable cardioverter defibrillator (“ICD”), also referred to as an implantable defibrillator, because the high voltage capacitors used to deliver the defibrillation pulse can occupy as much as one third of the ICD volume. ICDs typically use two electrolytic capacitors in series to achieve the desired high voltage for shock delivery.
One strategy for increasing energy density in the capacitor, and thus reducing its size, is to minimize the volume taken up by the paper and cathode and maximize the number of anodes. This may be achieved by using a multi-anode stack configuration. A multiple anode stack configuration requires fewer cathodes and paper spacers than a single anode configuration and thus reduces the size of the device. A multiple anode stack includes a number of units that contain a cathode, a paper spacer, two or more anodes, a paper spacer and a cathode, with neighboring units sharing the cathode between them. However, to charge and discharge the inner anodes (furthest from the cathode), charge must flow through the outer anodes. With a typical anode, the path through an anode is quite tortuous and results in a high equivalent series resistance (“ESR”) for the multi-anode configuration. Thus, ESR increases as more anodes are placed together in the stack. Another strategy for decreasing the size of the device is to increase the operating voltage of the capacitor, which may potentially allow for the use of only one electrolytic capacitor. The unique challenge of increasing the operating voltage of the capacitor, however, is that high voltage is usually correlated with a low surface area, which reduces capacitance and likewise energy (E=0.5*CV2).
Regardless of the particular strategy employed, metal foils (e.g., aluminum foil) have often been employed in the electrolytic capacitor due to their small size. Because the electrostatic capacitance of the capacitor is proportional to its electrode area, the surface of the metallic foil may be, prior to the formation of the dielectric film, roughened or subjected to a chemical conversion to increase its effective area. This step of roughening the surface of the metallic foil is called etching. Etching is normally carried out either by the method (chemical etching) of conducting immersion into a solution of hydrochloric acid or by the method (electrochemical etching) of carrying out electrolysis in an aqueous solution of hydrochloric acid. The capacitance of the electrolytic capacitor is determined by the extent of roughing (the surface area) of the anode foil and the thickness and the dielectric constant of the oxide film. Due to the limited surface area that may be provided by etching metallic foils, attempts have also been made to employ porous sintered pellets in wet electrolytic capacitors. A tantalum pellet, for instance, may be formed by compressing a powder under high pressure and sintering at high temperature to form a sponge-like structure, which is very strong and dense but also highly porous. The porosity of the resulting tantalum pellet provides a large internal surface area. Despite its high surface area, however, anode pellets may still present high ESR and sensitivity of the capacitance to frequency, particularly at the high voltages often encountered in medical devices. Further, the pellets are typically larger in size than the anode foils, thus making it difficult to incorporate them into application in which high volumetric efficiency is needed.
As such, a need currently exists for an improved electrolytic capacitor for use in implantable medical devices, such as defibrillators.